1. Field of the Invention
The present invention relates to a horizontal head with combined thin film write and MR (magnetoresistive) read elements at an air bearing surface (ABS) and more particularly to a merged or piggyback horizontal head wherein an MR sensor employs one or two MR stripes, the two MR stripes being uniquely formed for improved common mode rejection.
2. Description of the Related Art
A typical combined head includes a thin film inductive write element and a magnetoresistive (MR) read element. The thin film inductive write element includes one or more coil layers embedded in an insulation stack, the insulation stack being sandwiched between first and second pole piece layers that extend into a pole tip region. A gap layer forms a write gap between the pole pieces in the pole tip region. The pole pieces are magnetically coupled across a back gap in a back gap region. Between the pole tip region and the back gap region lies a yoke region where the pole piece layers separate from one another to accommodate the insulation stack. The insulation stack typically includes a first insulation layer (I1) on the first pole piece layer, one or more coil layers on the first insulation layer, a second insulation layer (I2) over the coil layer and a third insulation layer (I3) over the second insulation layer.
An MR read element includes an MR sensor sandwiched between first and second gap layers which are, in turn, sandwiched between first and second shield layers. In a merged head a single layer serves both as a second shield layer for the read element and as a first pole piece for the write element. In a piggyback MR head the second shield layer and the first pole piece are separate layers. The merged (or piggyback) head is carried on a slider which, in turn, is mounted on a suspension in a magnetic disk drive. The suspension is mounted to an actuator which moves the head over selected tracks on a rotating disk for reading and writing signals thereon. Rotation of the disk creates a cushion of air that serves as an air bearing between the disk and the slider that counterbalances a loading force exerted by the suspension. A surface of the slider facing the disk is called an air bearing surface (ABS). The ABS is typically spaced from the disk in the order of 0.050 μm when the disk is rotating. A combined head (that is, a merged or a piggyback head) may be a “vertical” head or a “horizontal” head. In a vertical head a major plane of the first pole piece layer is generally perpendicular to the ABS, with edges of the first and second pole piece layers exposed at the ABS. In a typical horizontal head horizontal components of the first and second pole piece layers form a portion of the ABS so that edges of these layers are generally perpendicular to the ABS and extend internally into the head without being exposed at the ABS. In a horizontal head an insulation or gap layer separates the edges of the first and second pole piece layers at the ABS.
In the vertical head, the MR sensor for the read element is located at the ABS. In the horizontal head the edge of the MR sensor for the read element is typically recessed from the ABS and receives read signals via one of the pole piece layers which serves as a flux guide. Accordingly, the MR sensor, the first and second shields and the first and second pole pieces are all in series. There are several problems with this arrangement. First, it is desirable that the trace of a track being read be narrower than the track as written. This is impossible with the prior art arrangement since the write gap also serves as the read gap. Next, each time a write operation is performed the shields are subjected to a high density of flux, which can render them unstable. As a result of instability, the magnetic domains of shield layers may not return to their initial state, which can change the bias point of the MR sensor and result in inaccurate playback.
In both the vertical and horizontal heads it is desirable to increase the signal-to-noise ratio durng readback. This can be accomplished by employing a dual stripe MR sensor wherein each MR stripe conducts an identical sense current. During operation, both sense currents may be conducted to a differential amplifier in order to implement common mode noise rejection. If the read head collides with an asperity on a magnetic disk, noise generated by this collision will be reduced by common mode rejection. However, it is difficult to obtain near absolute common mode rejection because the MR stripes are typically formed in separate process steps. When MR stripes are formed in separate process steps they are not identical, due to slight differences in temperature, pressure, atmosphere and process times. In a dual stripe, vertical MR head, the thin film layers of the read element are sequentially formed by separate process steps. Thus, there is a strong felt need to form the two stripes of an MR element in a single process step so that the two stripes are substantially identical, the better to implement near-absolute common mode rejection of noise.